As for the general fish getting along part of the question, it will really just depend upon if the fish like each other / get along. Sometimes they hate each other, some time best friends.
If you can think back to the fish personality, is the remaining fish more aggressive / most curious of the 2, leader as they swim. Or the follower, less curious. Try to find the opposite, of the remaining fih, or if the remaining fish is the follower possibly another follower to see if they have more chill behaviour and can get along alright.
And check out the link in my signature for getting started, regarding the nitrification cycle and having a good BB / beneficial bacteria to maintain healthy fish.
What size is the tank, filtration and the fish size? If you started another thread in unhealthy fish, please add those details there.
New to FW fish? First tank in a long time or EVER??? Need some info on keeping fish?
Start here - Aquarium Advice Article
FW Favorites right now Clown Loaches, any shrimp - SW Vase Red Mangroves